Exterior complex scaling method in TDDFT calculations: MPI and HHG of Ar atoms

Exterior complex scaling (ECS) method is applied in the framework of time-dependent density functional theory (TDDFT) to study multiphoton ionization (MPI) and high-order harmonic generation (HHG) of multielectron atoms in intense laser fields. ECS allows to impose correct (outgoing-wave) boundary conditions on the wave functions at large distances. In our implementation, ECS is combined with the time-dependent generalized pseudospectral method (GPS) for accurate and efficient solution of time-dependent Kohn-Sham equations. We make use of LB94 exchange-correlation potential which proved accurate in calculations of unperturbed electronic structure of Ar. Calculations of MPI and HHG are performed for the laser pulses with the wavelength 800~nm and several peak intensities. HHG spectrum exhibits an intensity-independent minimum corresponding to the photon energy of about 50~eV which is closely related to the Cooper minimum observed in photoionization cross section of Ar. We found that results obtained with the frozen-core potential (that is, not including dynamic response of the electron density to the laser field) significantly overestimate MPI probabilities as compared with those calculated by TDDFT.